The invention relates to a heat-conducting adhesive joint between two workpieces and a method for producing a heat-conducting adhesive joint between two work-pieces.
Electronic components, in particular power semiconductor components such as IGBTs, MOS-FETs, diodes, thyristors and so on produce high power losses during operation, which have to be dissipated efficiently in order not to exceed the maximum operating temperature.
For semiconductor chips up to at most 2xc3x972 cm in size, soft soldering with tin, lead and their alloys on supporting elements made of ceramic or metal has become completely widespread in the technology. Other methods, such as those with gold solders, glass pastes and so on have found only a very narrow field of use, for cost reasons.
Development trends are leading, on one hand, to ever higher operating temperatures until close to the melting points of the solders, with the reliability increased at the same time, but on the other hand lead should be superseded for environmental protection reasons, in particular also by law.
Adhesive bonding, which is otherwise very common in chip assembly technology, suffers from poor thermal conductivity and also poor electrical conductivity of the adhesives.
This poor thermal conductivity of the adhesives can be improved by means of particles which conduct heat well and which are suspended in the poorly conducting adhesive. For example, DE-A-195 29 627 (95P1762 DE) discloses improving the thermal conductivity of the adhesive by adding a heat-conducting powder, for example nickel powder.
In practical terms, this document describes a heat-conducting, electrically insulating adhesive joint between two workpieces, said joint having a layer of ceramic material and a layer of adhesive.
The layer of ceramic material has two flat surfaces which face away from each other and, on each flat surface, there are openings defined by voids in the layer, and the layer is arranged between the two workpieces in such a way that one of the two flat surfaces of one of the two workpieces, which is constructed in the form of a heat sink, has flat contact made with it. Furthermore, at least the openings on the other flat surface, which faces away from the one flat surface, is filled with electrically insulating material.
The layer of adhesive is arranged between the layer of ceramic material and the other workpiece, which forms an electronic power component, and has two flat surfaces that face away from each other. One of these surfaces makes flat contact with the other workpiece and adheres to the latter. The other surface makes flat contact with the other flat surface of the layer of ceramic material and adheres to said layer.
In order to improve the thermal conductivity of the layer of adhesive, a heat-conducting powder, for example nickel powder, is added to said layer.
This known adhesive joint is produced as follows:
The ceramic layer is produced on one workpiece by means of thermal spraying, the openings defined by voids in the layer being produced automatically on the flat surfaces of this layer.
At least the openings on the other flat surface facing away from the one flat surface and the one workpiece are filled with electrically insulating material.
The layer of the ceramic material which comprises the electrically insulating material is joined to the other workpiece by means of the adhesive layer to which the heat-conducting powder is added.
DE 34 14 065 A1 (84 P 1304) and EP 0 242 626 A2 (86 P 1242) in each case reveal a different type of adhesive-free joint between a workpiece in the form of an electronic component and a workpiece in the form of a substrate, which has a layer of heat-conducting material in the form of a sintered metal powder and is therefore both thermally conductive and electrically conductive.
The layer of the sintered metal powder has two flat surfaces which face away from each other and each of which has openings defined by voids in this layer.
The layer is arranged between the two workpieces in such a way that one of the two flat surfaces is sintered onto one of the two workpieces and the other flat surface is sintered onto the other workpiece.
The sintered metal powder of the layer is coherent from one of the flat surfaces in the direction of the other flat surface.
The production of the adhesive-free joint according to DE 34 14 065 A1 is carried out by the following steps:
A paste is applied to one workpiece and/or the other workpiece, said paste being composed of a mixture of a metal powder which can be sintered at a specific sintering temperature and a liquid.
The two workpieces are brought together in such a way that the paste is located between the two workpieces and makes contact with both workpieces.
The paste is dried and the dried powder is sintered by heating to the sintering temperature. This sintering is carried out in a non-oxidizing atmosphere, for example N2 or H2, and the sintering temperature in this case is about 400xc2x0 C. During the sintering operation, a mechanical pressure, for example 80 to 90 N/cm2, can be exerted.
The production of the adhesive-free joint according to EP 0 242 626 A2 is carried out by the following steps:
A paste is applied to a workpiece, said paste being composed of a mixture of a metal powder that can be sintered at a specific sintering temperature and a liquid.
The paste is dried.
The other workpiece is placed on the dried powder.
The entire arrangement is then heated to sintering temperature with the simultaneous exertion of a mechanical pressure of at least 900 N/cm2. The sintering temperature is about 230xc2x0 C. to 250xc2x0 C.
In the thesis by Sven Klaka: xe2x80x9cEine Niedertemperatur-Verbindungstecknik zum Aufbau von Leistungshalbleiter-modulenxe2x80x9d [A low-temperature joining technique for the assembly of power semiconductor modules], Cuvillier Verlag, Gxc3x6ttingen 1997, in this connection the sintering operation in the case of silver powder is examined at low sintering temperatures between 100xc2x0 C. and 250xc2x0 C. and it is established that this powder can form sintered bridges between 200xc2x0 C. and 250xc2x0 C.
The invention is based on the object of providing a heat-conducting adhesive joint between two workpieces which exhibits a higher thermal conductivity than a joint with a layer of adhesive to which a heat-conducting powder is added.
This object is achieved by the features of claim 1.
According to said claim, the heat-conducting adhesive joint according to the invention comprises:
a layer of heat-conducting material,
which has two flat surfaces facing away from each other,
which, on each flat surface, has openings defined by voids in the layer,
which is arranged between the two workpieces in such a way that one of the two flat surfaces (31) makes flat contact with one of the two workpieces, and the other flat surface makes flat contact with the other workpiece, and
whose heat-conducting material is coherent from one of the flat surfaces in the direction of the other flat surface,
and
an adhesive
which fills the openings in the layer and
which adheres to both workpieces.
The term xe2x80x9ccoherentxe2x80x9d is to be understood in such a way that, in the layer of heat-conducting material, this material coheres from one of the flat surfaces in the direction of the other flat surface of the layer outside the voids of this layer, or forms a unit, at least in such a way as in the case of a layer of sintered powder of heat-conducting material. Such coherence, beneficial for the thermal conductivity, is not present in a thermally and electrically poorly conducting layer of adhesive to which heat-conducting powder is added, since heat-conducting paths are formed in this layer only at comparatively few points of contact between the particles of the added powder.
The higher the thermal conductivity of the heat-conducting material of the layer, the more beneficial this is for the heat-conducting adhesive joint according to the invention.
One advantage of the adhesive joint according to the invention is that it can be implemented as desired as an electrically conductive or electrically insulating joint, depending on whether the heat-conducting material selected for the layer is electrically conductive, for example metal, or electrically insulating, for example heat-conducting ceramic material.
The strength of the adhesive joint according to the invention is advantageously composed of the inherent strength of the layer of heat-conducting material and the strength of the adhesive, can therefore become significantly greater than in the case of an adhesive joint of pure adhesive or adhesive to which powder of heat-conducting material is added. At high temperatures, the strength of the layer of heat-conducting material generally dominates. The adhesive advantageously protects the layer of heat-conducting material, in particular at high temperatures, against reaction of the layer with oxygen or another oxidizing gas.
In principle, it is sufficient if the layer of heat-conducting material has only voids which define openings on the flat surfaces of the layer. For example, the layer can be a film of heat-conducting material with holes, each of which defines one opening on both flat surfaces of the layer.
The layer of the heat-conducting material is preferably interspersed in the manner of a sponge with voids, so that there are also voids in the interior of the layer which do not immediately adjoin the flat surfaces of the layer and do not define any openings on these surfaces.
In this case, it is advantageous if at least those voids in the layer which define openings on one flat surface of the layer are joined to one another. Through these voids, curable liquid adhesive can be introduced in a simple way from the outside into the openings on one flat surface of the layer, even if this surface is already in contact with a workpiece.
It is beneficial if as far as possible all the available voids are connected to one another, and, in relation to the curable liquid adhesive, are so small that, for this adhesive, they act like capillaries which exert a suction action on said adhesive. In this case, the curable liquid adhesive can advantageously substantially be introduced automatically from the outside through the layer and into the openings on the flat surfaces of this layer by means of capillary suction action, irrespective of whether the surfaces are already in contact with a workpiece or not. As an alternative or in addition to the capillary suction action, the curable liquid adhesive can be introduced into the layer with support from pressure.
In a preferred embodiment of the joint according to the invention, the heat-conducting material of the layer is selected from the group of metals, in particular from the group of noble and semi-noble metals. It is particularly advantageous here for the heat-conducting material to comprise silver.
The layer is preferably and advantageously composed of sintered metal powder. Such a layer, which is electrically conductive, has the following advantages, for example: it can be produced easily, under certain circumstances it can be sintered onto a workpiece or onto both workpieces to be joined and, on its own, can already form a heat-conducting joint to one or both workpieces, which assists the joint produced by the adhesive and is inherently formed in such a way that it has openings defined by voids on its flat surfaces and is interspersed in the manner of a sponge with voids, the voids being connected to one another and, in relation to a curable liquid adhesive, being capable of being so small that they exert a capillary suction action on this adhesive, and so on.
The adhesive joint according to the invention is particularly well suited for fixing an electronic component, in particular a power component, to a supporting element, that is to say, in the case of this joint, one workpiece is the electronic component, in particular the power component, and the other workpiece is the supporting element for the electronic component. The supporting element preferably comprises a heat sink for the electronic component.
The invention also provides a method for producing a heat-conducting adhesive joint between two workpieces, which has a higher thermal conductivity than a joint with a layer of adhesive to which a heat-conducting powder is added, and which comprises the steps of:
producing a layer of heat-conducting material,
which has two flat surfaces facing away from each other,
which, on each flat surface, has openings defined by voids in the layer,
which is arranged between the two workpieces in such a way that one of the two flat surfaces makes flat contact with one of the two workpieces, and the other flat surface makes flat contact with the other workpiece in each case, and
whose heat-conducting material is coherent from one of the flat surfaces in the direction of the other flat surface,
introducing liquid curable adhesive into the openings in the layer arranged in this way between the two workpieces, such that the liquid adhesive introduced wets each workpiece and
curing the adhesive introduced in this way.
According to this method, first of all a highly thermally conductive layer is produced which is in contact with both workpieces, and only then is the layer bonded adhesively to the workpieces.
The heat-conducting layer is preferably and advantageously produced by the steps of:
applying a paste to a workpiece and/or the other workpiece, said paste being composed of a mixture of a powder of heat-conducting material that can be sintered at a specific sintering temperature and a liquid,
bringing the two workpieces together in such a way that the paste is located between the two workpieces and makes contact with both workpieces,
drying the paste and
sintering the dried powder by heating to the sintering temperature.
The layer of sintered powder can advantageously be electrically conductive or nonconductive, depending on the selected heat-conducting material of the powder and, moreover, can have the same advantages as are described above in relation to the layer of sintered metal powder, that is to say it can be produced easily, under certain circumstances it can be sintered onto one workpiece or onto both workpieces to be joined and, on its own, can already form a heat-conducting joint with one or both workpieces, which assists the joint produced by the adhesive, it is inherently formed in such a way that it has openings defined by voids on its flat surfaces and is interspersed in the manner of a sponge with voids, the voids being connected to one another and, in relation to a curable liquid adhesive, being capable of being so small that they exert a capillary suction action on this adhesive, and so on.
A higher density and therefore higher thermal conductivity of the sintered layer of the heat-conducting powder can be obtained if very much finer and/or very much coarser powder made of heat-conducting material is added to the powder. Coarse-grained powder can be composed of metal or other substances with a good thermal conductivity, for example of SiC or diamond.
A high density and therefore good thermal conductivity of the sintered layer of heat-conducting material can also be achieved with the step of exerting a specific mechanical pressure on the powder during the sintering operation or after the completion of this operation.
Use is preferably made of a sinterable powder selected from the group of metals, in particular of the noble and semi-noble metals.
It is particularly advantageous to make use of a sinterable powder comprising silver. If use is made of a powder comprising silver particles, and the sintering of this powder is carried out in an oxidizing atmosphere, a sintering temperature between 100xc2x0 C. and 250xc2x0 C. is advantageously adequate for sintering. The sintering in an oxidizing atmosphere can also be advantageous in the case of sinterable powders which contain substances differing from silver.